1. Field of the Invention
The invention relates to an electrical filter circuit, operated with a definite sampling and clock frequency f.sub.T, made up of CTD elements and incorporating at least one bipolar or quadripole resonator in the form of a self-contained conductor loop with unidirectional transmission behavior.
2. The Prior Art
Filter circuits and resonator circuits of the above type in the form of one port or two port resonators have already become known from the copending applications of Friedrich Kuenemund, Serial No. 630,932, filed Nov. 11, 1975, now abandoned, and Serial No. 151,772, filed May 21, 1980, now U.S. Pat. No. 4,393,356. The disclosure of the Kuenemund patent is incorporated hereinto by reference. It is described in that patent that so-called BBD arrangements (bucket brigade devices) so-called CCD arrangements (charge coupled devices) are to be understood by CTD lines. CCD circuits are such devices as function according to the principle of coupled charges. The designation "CTD device" (charge transfer devices) has become standard as a superordinate term for these two types of circuits and it is characteristic of said CTD lines that they consist of a greater plurality, thus, for example, n individual CTD elements which can be realized as fully integratable overall arrangements. As is known, such CTD arrangements must be operated via a clock signal with a predetermined clock frequency f.sub.T, with the clock signal being supplied to the individual transfer capacitors. In practice, so-called multi-phase CTD arrangements are preferred, their clock signals being phase-shifted with respect to one another in such manner that neighboring transfer capacitances are driven with phase-shifted clocks. If one employs a so-called p-phase clock system (p=2, 3, 4 . . . ), then one CTD element consists of p neighboring transfer capacitances. Details concerning this may be found, for example, in the book "Charge Transfer Devices", Academic Press Inc., New York, San Francisco, London 1975. Circuit possibilities are also cited in said book and in the other publications which have been referred to as to how signals for the further processing in a CTD arrangement can be edited or, how signals transmitted via CTD arrangements can be reconverted into other signal forms, such as analog signals. In the German Letters Patent already cited, filter circuits are specified in which such CTD lines are wired into a self-contained ring capable of resonance, whereby the resonant frequency of said rings is directly determined by the plurality n of the CTD line elements employed for the self-contained conductor loop and the clock frequency. Codeterminate for the filter characteristic is the ratio of the transfer capacitances of the CTD arrangement employed in the output line to the transfer capacitance of the CTD arrangement employed in the closed conductor loop. The arrangement illustrated and described in the Kuenemund patent incorporates a plurality of CTD lines, each having plural storage cells and interconnected so as to have unidirectional transmission behavior, with one of the CTD lines connected as a closed loop to serve as a resonator.
CTD filter circuits according to the Letters Patent cited above with a plurality of bipolar or quadripolar resonators connected in a series have high values for the capacitance ratios in the bipolar or quadripolar resonator, given band pass circuits with the relatively small pass band: B=.DELTA.f.sub.3 db /f.sub.m &lt;1%.
Thus, the transfer capacitances for the feeder or, respectively, tap lines differ relatively greatly from the transfer capacitances of the CTD lines to be employed in the closed conductor loops.